THE INTERNAL ORGANS OF A FLEA
A FLEA like every other animal must feed and breathe, which leads to
a consideration of the internal organs of digestion and respiration.
The digestive canal is a slender tube which connects the mouth and the
anus, and which is less convoluted and much straighter than in the
higher vertebrates. Fig. 6 will show the relative positions of the
various parts, namely, the mouth, pharynx, gullet, gizzard, stomach,
and rectum. Connected with the digestive canal are certain glands and
organs of excretion. The alimentary tube itself passes through the
middle of the flea’s body, and is kept in that position partly by
muscles and partly by the numerous branching air-tubes through which
the insect breathes. Above it lies the heart, and beneath it the
nervous cord or chain of ganglia.
[Illustration: Fig. 6. Diagram of the alimentary canal of a flea.
At the top is shown the orifice of the _mouth_, leading into the
_pharynx_. Next comes the short _gullet_. The _gizzard_ is the smaller
organ immediately before the stomach. At the base of the _stomach_ are
four vermiform tubes, which are the _Malpighian tubules_. From the base
of the stomach issues the _intestine_, which leads to the _rectum_,
where the six _rectal glands_ are shown.]
The mouth of a flea, as of any other insect, is merely an orifice which
forms the opening into the alimentary canal. Around the orifice are the
various mouth-parts which convey blood to the mouth, but these, the
reader will doubtless remember, are the modified limbs or appendages
of the segments that compose the flea’s head. The mouth, then, gives
access to the digestive canal. The first part nearest the mouth is the
pharynx which merges gradually into the gullet. Here is placed the
pharyngeal pump which is provided with a sucking apparatus. Muscles
attached to the dorsal part of the so-called aspiratory pharynx cause
it to expand and contract, owing to the elastic reaction of its walls.
The operating muscles, which do this, are in the head of the flea.
When these pharyngeal muscles contract and relax in regular sequence,
a rhythmic action of the pharynx itself ensues and a steady stream
of blood is forced or drawn from the mouth stomachwards. In a light
coloured flea, under a powerful lens, this action may be watched in the
living insect.
Behind the pharynx comes the gullet, which leads down to the gizzard.
It is perhaps needless to add that this organ, neither in appearance
nor in use, bears any resemblance to the gizzard of a bird, which
grinds hard food. The food of the adult flea consists solely of liquid
blood.
The organ called _gizzard_ in the flea, for want of a better name,
is, however, remarkable. Its function is not quite certainly known.
It is a bulbous expansion in the front of the stomach and situated at
the junction of the stomach and the gullet. It contains a multitude of
chitinous finger-like processes tapering towards their extremities.
From their general arrangement the complete collection of processes
would act as an effective sort of valve and prevent the return of
the fluids from the stomach. It seems most probable that this is
their function. During the life of the flea the stomach is constantly
churning its contents. Some valvular arrangement between the stomach
and the pharynx would seem to be essential; the pharynx is normally
collapsed, as the reader may remember, and its walls are drawn apart by
muscles attached to its exterior. When the pharynx is full of blood the
muscles relax, the walls collapse like elastic, and the blood is forced
into the stomach. In many cases a flea will feed when the stomach is
already tensely full of blood; and some sort of valve is therefore
needed to prevent regurgitation into the pharynx when the pharyngeal
muscles contract and the walls of the pharynx itself are drawn asunder.
This valvular arrangement at the anterior end of the flea’s stomach has
been minutely studied in connection with recent plague investigations,
because there was a theory that fleas carried infection by vomiting
the septicæmic blood from their stomachs and so transferred the plague
bacillus to the puncture which they made in the skin.
But an experiment, which has been tried several times, seems to show
that the supposed valve is effective. The stomach of a flea which had
recently fed was dissected out intact. As long a portion of rectum as
possible was left attached at the hinder end. The gullet having been
severed, well in front of the valve, pressure was applied with a blunt
tool with the object of forcing the blood through the gullet. The hind
aperture of the stomach was, at the same time, closed by pinching
up the rectum. The result was that, in no instance, was it possible
to force blood through the passage which leads into the gullet. Yet
sufficient pressure was applied to burst the stomach.
The stomach of a flea is a pear-shaped sack which occupies an
appreciable part of the insect’s abdomen. That it is capable of
containing a comparatively large amount of blood is apparent from the
observation that after a flea has enjoyed a good meal nearly the whole
of the abdomen is seen to be filled with a bright red mass. During
the investigation of the part played by fleas in spreading plague an
endeavour was made to measure, as accurately as possible, the average
capacity of a rat-flea’s stomach when filled with blood. Healthy fleas,
taken from Bombay rats, were starved for twelve hours, and at the
end of that time were fed on healthy animals. The stomach was then
dissected out whole and floated in a salt solution. Any adherent organs
or muscles were carefully removed. Under these conditions the stomach
can be examined and measured under the microscope. The average capacity
of a rat-flea’s stomach has been approximately estimated to be half a
cubic millimetre.
The stomach of a flea is therefore, comparatively speaking, very large.
The blood remains in the stomach in a partially digested condition.
It gradually diminishes in volume, showing clearly that absorption
is taking place. At the end of so much of the digestive process as
takes place in the flea’s stomach, the blood has become reduced to a
thick, slimy, dark red mass. This passes down the intestine to the
rectum, where it is perhaps further influenced by the secretion of the
so-called rectal glands. Finally, the undigested remains pass from the
rectum in the form of very minute, round, almost black, tarry drops.
The terminal section of the flea’s digestive canal is called the
rectum. Here are placed the rectal glands (Fig. 6), which are six in
number. Their function seems not to be certainly known.
The external opening of the rectum is placed at the extreme end of the
flea’s body between the tergite and sternite of the tenth segment.
We pass now to a couple of quite distinct appendages of the digestive
canal, namely the salivary glands and the urinary tubules. In fleas
the salivary glands are four in number. Two are placed on each side of
the anterior end of the flea’s stomach. Each is a simple acinous gland
embedded in the body and lined with cells which secrete the saliva.
The four ducts from the pairs of glands unite to form two ducts; and
the two ducts thus formed run forward and open into the salivary pump.
A spiral chitinous membrane lines the inside of the ducts, keeps them
distended, and gives them somewhat the appearance of tracheal tubes.
The salivary pump is placed quite in the front part of the insect’s
head, and is an organ worthy of special notice. It receives the saliva
from the glands by means of the two salivary ducts which have just
been described, and propels it through the exit duct of the pump into
the salivary canal in the mandibles. The pump itself is a hollow
chitinous organ. Muscles attached to the walls alternately contract
and relax, drawing up the salivary secretion and expelling it through
the exit-duct. The opening of the exit-duct is adjusted so as to be
opposite to the canals which extend down the mandibles like troughs.
It would seem that when the flea is feeding, saliva is pumped into the
puncture and blood is pumped out. There is, as it were, an effluent and
an affluent stream passing along the mouth parts.
The urinary tubules are excretory organs which carry off, in solution,
the waste products of the flea’s body. They are sometimes also called
Malpighian tubes (Fig. 6). This name they received after Malpighi
(1628-94), a famous Italian anatomist, who, four years after Harvey’s
death, saw with his own eyes the capillary circulation of which Harvey
had only inferred the existence. He also was the first to detect the
urinary tubes of insects. These tubules answer to the kidneys of the
higher vertebrate. They vary in number in different insects from two
to over a hundred. In fleas there are four. They are longish, slender,
tubular glands which are closed at one end, but, at the other, open
into the rectum. The urinary excretions come from the blood, pass down
the tubes into the rectum, and so leave the flea’s body by the anus. In
insects the urinary excretion is, generally, only partially liquid.
The organs of respiration in a flea consist of a series of tracheæ,
or air-tubes, which open by apertures, called stigmata, at the sides
of the body. These air-tubes branch and form an elaborate system
of ramifications. They have a horny lining and are supported by a
spirally-wound thread-like thickening. In this way air is conveyed from
the external world, and the oxygen, which vital processes require, is
conducted to all parts of the insect’s body.
The blood-system of a flea is far less complete than that of the
lowest vertebrate. The blood is almost colourless. A large contractile
heart drives it into the main blood-vessel. There is, however, no
closed system of arteries, capillaries, and veins such as the higher
animals possess; and the blood circulates in the whole cavity which
intervenes between the body-wall and the various internal organs. There
is little need for an elaborate system of blood-vessels since the
internal tissues are supplied with oxygen by the ramifying air-tubes.
Fleas have more of the air-holes called stigmata than any other
insects. Each of the three segments of the thorax has a pair, as well
as the second to the eighth segments of the abdomen. The spiracles or
apertures lie free on the outside of the body. In beetles, and other
insects which run through dusty places, they are lodged in the thin
membrane between the segments.
The heart of a flea is a very delicate pulsating tube which lies
along the back, above the digestive canal and immediately beneath the
integument. One may attribute some of the extraordinary strength and
vital energy of a flea to the fact that, by the blood-system and the
air-system, the tissues of the body are kept richly supplied with
oxygen. The blood of a flea is a thin fluid and, of course, without red
corpuscles. The blood that is shed when a flea is crushed comes from
the stomach and not from the blood-vessels of the insect.
The internal organs of fleas cannot be studied without dissection
under a microscope. Dissection is best carried on in a solution of
salt and water. Fine needles mounted in penholders are the most handy
implements. But the point of even the finest commercial needle that can
be bought is too blunt for fine dissections, and it is necessary to
sharpen it. This can be done by the help of a rapidly revolving emery
wheel, varying the inclination of the needle-point to the wheel, so
as to grind off the angles. The flea to be dissected is put in a drop
of salt solution, on a slide placed on the stage of the dissecting
microscope. In the left hand should be a needle with a blunt conical
point, in the right a needle with an oblique point. The antennary
groove of the flea should then be transfixed and held firmly by the
left-hand needle.
The point of the right-hand needle is then inserted under the edge of
the third or fourth abdominal segments. The segments can then be peeled
off by a skilful dissector much as we peel off the skin of a shrimp for
our tea at the sea-side. The internal organs of the flea then float off
in the salt solution; and by using two very fine pointed needles they
can be further separated. It is useful to have one needle ready with a
hooked end and another fashioned into a minute knife or scalpel.
The most conspicuous of the internal organs will be the stomach and
intestine. The salivary glands will be found at the side of the stomach
with a certain amount of fat round them. Their extraction is not so
difficult as might be supposed. The hooked needle can be used to hook
the salivary duct.
The most difficult parts to dissect are the organs connected with the
mouth and rostrum. It is best to remove the head and transfix it with
the left-hand needle, then to scalp the head by removing the dorsal
half of the chitinous carapace. A bold plunge with the right-hand
needle will sometimes effect what is desired. A pull on the labium will
sometimes bring out the pharynx. It must be confessed that successful
dissections are often obtained more by good luck than by skilful
management. The use of dilute potash solution facilitates the study of
chitinous parts by jellifying the muscles.